Chemical etching of a silicon wafer is performed for removing a damage layer (alteration layer due to processing) on a surface of the silicon wafer caused mainly by a mechanical processing step, such as block cutting, outer layer grinding, slicing and wrapping, and is also performed on the back surface in some cases, so that front and back of a silicon wafer, wherein both sides are mirror polished, can be identified.
In chemical etching for removing a damage layer, an acid etchant or an alkali etchant is used as the etchant. The former is an etchant formed by three components, wherein mixed acids of fluorinated acid (HF) and nitric acid (HNO3) is diluted by water (H2O) or by acetic acid (CH3COOH). Silicon Si is oxidized by the nitric acid to generate SiO2, then, the SiO2 is dissolved by fluorinated acid and removed. The latter etchant is an etchant obtained by diluting potassium hydroxide (KOH) or caustic soda (NaOH), etc. by water. As a result of such chemical etching, a processing alteration layer of a silicon wafer is removed, so that the surface roughness is reduced and a smooth surface is obtained.
However, the conventional etchant as explained above has not been able to uniformly lower the gloss level of a mirror surface of a mirror finished silicon wafer while maintaining a global flatness level, for example, TTV (total thickness variation: a difference (μm) of a maximum value and a minimum value of wafer surface height when fixing the wafer back surface by suction; GBIR of the SEMI standard).